Time-of-Flight Mass Spectrometers (TOF-MS) are devices used to analyze ions with respect to their ratio of mass and charge. In a typical linear TOF-MS, as it is described e.g. in U.S. Pat. No. 2,685,035 and Wiley et at., ions are accelerated in vacuum by means of electrical potentials which are applied to a set of parallel, substantially planar electrodes, which have openings that may be covered by fine meshes to assure homogeneous electrical fields, while allowing the transmission of the ions. The direction of the instrument axis shall be defined as the direction normal to the flat surface of these electrodes. Following the acceleration by the electrical fields between said accelerator electrodes, the ions drift through a field free space or flight tube until they reach the essentially flat surface of an ion detector, further referred to as a detector surface, where their arrival is converted in a way to generate electrical signals, which can be recorded by an electronic timing device. An example of such a detector is a multi channel electron multiplier plate (MCP). The measured flight time of any given ion through the instrument is related to the ion's mass to charge ratio.
In another typical arrangement (See e.g., U.S. Pat. No. 4,072,862, Soviet Union Patent No. 198,034, and Karataev et al., Mamyrin et at.), the motion of the ions is turned around after a first field free drift space by means of an ion reflector. In such a Reflector-TOF-MS the ions reach the detector after passing through a second field free drift space. The properties of such ion reflectors allow one to increase the total flight time, while maintaining a narrow distribution of arrival times for ions of a given mass to charge ratio. Thus, mass resolution is enhanced over that of a linear instrument.
It is common practice to use electrostatic deflectors with homogeneous fields in TOF-MS in order to steer the ions towards the detector. In one particular case, this is done in order to offset a common perpendicular component of motion of the ions prior to the acceleration. In another case, deflectors are employed in order to establish a V shaped configuration of accelerator, reflector and detector in a Reflector-TOF-MS. Traditionally, the steering action required has been small and its impact on the mass resolution of the instrument has been neglected (Karataev et al., Mamyrin et at.).
Recently, however, new atmospheric pressure ionization techniques, which are especially well suited for the ionization of complex biomolecules, have renewed the interest in the orthogonal injection of eternally generated ions into the accelerator of a TOF-MS. This method was originally described by O'Halloran et at.; recent implementations are found in Dawson et at., Dodonov et at., Verentchikov.
In this particular application of TOF-MS, the injected ions can have substantial kinetic energy and, hence, a substantial velocity component perpendicular to the flight tube axis. The result of this velocity component is an unwanted oblique drift of the ions in the flight tube of the mass analyzer. It follows that a relatively strong steering action is required to redirect the ions towards the instrument axis and the detector. It was found experimentally that such steering causes distortions in the distribution orion flight times which can considerably diminish the mass resolution of the instrument.
The present invention recognizes the physical reasons for distortions created by the steering of the ions, and corrects these distortions by mechanically adjusting the detector surface at a calculated angle that enhances the mass resolution of the instrument.